1. Field of the Invention
This invention relates to methods and apparatus for analyzing turbid media by means of frequency-domain photon migration measurements and steady-state reflectance measurements. While not limited thereto, this invention is particularly useful for analyzing human body tissue in vivo. As one example, this invention is useful for analyzing female human breast tissue for detecting abnormal tissue conditions.
2. Description of the Prior Art
Reflectance spectroscopy is a technique for characterizing turbid media that has become widely used in medical diagnostics. In many cases quantification of chromophore concentrations is desired, and this requires the ability to separate the effects of absorption from those of scattering. Fundamentally, the coefficient of absorption ma and the coefficient of reduced scattering μs′ can be determined by a series of reflectance measurements performed in one of three domains, namely, time (with a fast pulse of light), frequency (with a sinusoidally modulated source of light), and steady state (with a source of constant intensity but multiple detectors at different distances).
Unsurprisingly, these three techniques have different merits and limitations. Spatially resolved steady-state techniques are relatively inexpensive and are more readily suited for the determination of μa and μs′ over large, continuous ranges of wavelengths than are the other methods. However, the steady-state approach works best when measurements are performed with a combination of short (˜1 transport mean free path) and long (many transport mean free paths) source-detector separations. Ideally, the optical properties of the sample should not vary over the ranges of volumes probed by the various measurements. The larger the spread of distances probed, the more likely that heterogeneities, such as those found in biological tissue, will distort the data from the predictions of the model. One approach to limiting this effect, given that the shortest separations provide great stability for the calculation of μs′, is to use relatively short (<10 mm) source-detector separations. Inasmuch as the mean probing depth scales with the source-detector separation, with this approach such measurements are sensitive to superficial components (to depths of less than 5 mm for typical biological tissues).
Time-domain and frequency-domain techniques are well suited for deeper (>1 cm for biological tissue) investigations. Moreover, they can be performed with only one or a few source-detector separations, which makes them more robust for use in studying heterogeneous samples. Because such techniques require sources that can be pulsed or modulated rapidly, covering a large wavelength range requires a tunable laser or an extensive collection of laser diodes, both of which can be expensive, difficult to maintain, and slow to cover the entire spectrum. This is an important drawback, because the quantification of chromophore concentrations can be significantly affected by use of a limited number of wavelengths.